This invention relates to an electromagnetic induction cooking apparatus wherein a substantially constant input power can be supplied to an induction heating coil. More particularly, it relates to an electromagnetic induction cooking apparatus in which a high-frequency current is supplied to an induction heating coil, thereby generating and applying a high-frequency magnetic field to a cooking utensil such as a pan or a kettle, and thus generating an eddy current, whereby the cooking utensil generates heat for cooking due to a loss of eddy current. The input power to the heating coil control is controlled to be substantially constant, irrespective of the material of the cooking utensil.
The conventional electromagnetic induction cooking apparatus comprises a top plate, an induction heating coil and an inverter. The coil is provided below the top plate and constitutes a series LC resonant circuit. A pan (i.e., a load) can be put on the top plate. The inverter supplies the coil with a high-frequency current of 20 to 30 KHz. The coil generates and applies a high-frequency field to the pan placed on the top plate, whereby an eddy current flows in the pan, to thereby generate heat for cooking. The eddy current concentratedly flows through the induction heating coil due to the skin effect. Therefore, the eddy current depends largely on the skin resistance Rs of the pan. It also depends on the skin depth .delta. and the specific resistance .rho. of the pan material. The values of Rs and .delta. are given by ##EQU1## Where "f" is frequency, and .mu.s is specific permeability. As seen from the equations (1) and (2), the skin depth .delta. and skin resistance Rs are determined by the specific resistance .rho. of the material and the specific permeability .mu.s of the pan.
The following talbe shows the values of .rho., .mu.s, Rs and .delta. for materials of pans used in this kind of electromagnetic induction cooking apparatus.
__________________________________________________________________________ Pan Material Quality Non-magnetic Characteristic Value Iron Stainless steel Aluminum Copper __________________________________________________________________________ Specific Resistance .rho.(.OMEGA./m) 17 .times. 10.sup.-8 70 .times. 10.sup.-8 2.8 .times. 10.sup.-8 1.7 .times. 10.sup.-8 Specific Permeability .mu.s 100 1 1 1 Skin Resistance Rs(.OMEGA. ) 1.3 .times. 10.sup.-3 0.27 .times. 10.sup.-3 0.053 .times. 10.sup.-3 0.041 .times. 10.sup.-3 Skin Depth .delta.(m) 0.13 .times. 10.sup.-3 2.6 .times. 10.sup.-3 0.53 .times. 10.sup.-3 0.41 .times. 10.sup.-3 __________________________________________________________________________
As seen from the table, the material of the pan, such as a ferromagnetic material with a high specific permeability, such as iron or magnetic stainless steel, or non-magnetic stainless steel with a high specific resistance, has a large skin resistance. Therefore, the heating coil has a large input impedance. In this respect, there is no problem in realizing the induction heating.
When the material of the pan has a high permeability, like iron, the skin depth s is low, the skin effect is large, and the pan itself assumes a high resistance. As a result, the input impedance of the heating coil is high. When the pan is made of non-magnetic (18-8) stainless steel, its specific permeability .mu.s is low and 1, and therefore the skin depth .delta. must be small theoretically. However, because the thickness of an actual pan is small, the resistivity .rho. value is more effective. The resistance of the pan itself increases with this high resistivity .rho., and the input impedance of the heating coil increases.
As shown in the table above, in the case of aluminum or copper pans whose specific permeability and specific resistance are very small, the skin resistance is very small and the input impedance of the heating coil is small. Therefore, a large current such as short-circuit current flows, but the problem of inability to heat arises. In other words, if the pan material is aluminum or copper, the specific permeability .mu.s is 1 and is thus small. The skin depth .delta. is large, and the skin effect is difficult to produce. Especially the resistivity .rho. is also small. The pan itself will be of low resistance. As a result, the input impedance of the heating coil is decreased.
It is possible to further increase the frequency of the high frequency current, in order to solve this problem, but it would have to be increased up to several MHz. Also, this large an increase in the frequency is practically impossible, from the standpoint of the operating speed characteristics of the switching element of the inverter. Supposing that it were possible, since the frequency would be extremely high, the actual resistance caused by the skin effect in the inductance heating coil would be increased suddenly, and the problem of extremely reduced efficiency would arise.
In other words, in the case of a copper or aluminum pan, if the high-frequency field is intensified, the input impedance of the heating coil can be theoretically raised to a value approximately equal to a value resulting when an iron pan is used. Hence, the coil can heat the pan sufficiently. However, the frequency f of the high frequency magnetic field must be intensified several hundred times over that with an iron pan, and from the standpoint of the switching element used in the inverter, actual realization is very difficult. Specifically, the frequency of the magnetic field of an inductance heating cooker is an audible frequency of above 18 kHz. If this is the frequency of the high frequency magnetic field of an iron pan, if an aluminum or copper pan is to be used, a high-frequency magnetic field with a frequency as high as several MHz must be generated, and the increase of losses in the heating coil and the inverter is incurred.
To qualitatively explain the above information, in a home use type electromagnetic induction heating cooker, in order that an input around 1.2 KW can be used, the switching element of the inverter must have a current capacity of several tens of amperes and a withstand voltage of several hundred volts. The time required for a switch element (transistor) to turn ON and OFF, that is to say, the accumulation time t.sub.stg +fall time t.sub.f, is about 1-2 .mu.s, but the period for 1 MHz is 1 .mu.s. In consideration of this fact, the construction of a multi-MHz inverter, as described above, is impossible.
When a high-frequency current flows in a heating coil made of copper wire, if the current density of the copper wire is uniform, the flux density at the center inside the copper wire increases. The longitudinal voltage distribution in the copper wire must be the same as the voltage distribution on the surface and at the center inside the copper wire. To make the longitudinal voltage distribution uniform, it is necessary for the flux density distribution to be uniform. Therefore, it is necessary to reduce the flux density in the center. This is the cause of the skin effect in the heating coil. When the copper element wire is twisted to form a coil, according to the same principle, the current inside the element wire becomes non-uniform due to the flux of other element wires. This is known as the "proximity effect." Accordingly, if the number of turns of the heating coil is increased, the losses that depend on the unit length of the heating coil increase.
For the above reasons, since the real resistance of the heating coil at high frequencies increases as the current frequency increases, the losses that occur in the heating coil increase. When the frequency is increased greatly, most of the inverter output power is consumed in the heating coil, power will not be applied to the pan, and the efficiency of the cooker will be very low. Also, the heating coil temperature will become very high becuase of the losses, and the problem of deterioration of the insulating covering of the copper wire will arise.
For the above reasons, the skin resistance of aluminum similarly can be increased by merely increasing the frequency of the heating coil.
The above approach, if taken to increase the input impedance of the heating coil, also increases the frequency and the skin resistance of the pan. There is another approach to increase the input impedance, however, in which the number of turns of the heating coil is increased. However, by simply doing this, the heating coil impedance is increased, and the resonant frequency is lowered. Therefore, in order to maintain the resonance of inverter load, which is composed of the heating coil and resonant condensers, the resonant frequency of the inverter must be lowered. When the frequency is lowered, not only does the skin resistance of the pan decrease, but the skin depth .delta. increases, and the problem that the input impedance of the heating coil changes occurs, depending on the thickness of the plate under the pan. In this case, with the frequency decrease, the skin resistance becomes still lower, and the number of turns of the heating coil must be increased further. As a result, the structure of the coil becomes even more complicated.
As shown above, with earlier techniques of making inductance heating cookers, the use of aluminum or copper pans to achieve the same efficiency and power input ultimately is impossible. The above facts thus show that there is no point in judging the pan material, based only on its magnetic or non-magnetic properties.